The belts of such fusers typically have an outer layer of material which tends to release toner from the belt, as the objective is for the belt to heat the toner while leaving it on the paper or other media being operated upon. Typically this outer layer is a low surface energy polymer, such as PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy vinyl polytetrafluoroethylene copolymer) or a physical blend of PTFE and PFA. Such a layer is particularly subject to damage and removal by wear.
Adequate lubrication reduces the force to move the belt and therefore reduces wear on the outer layer. However, a lubricant having high viscosity at cold and low standby temperatures is employed because a less viscous lubricant, such as an oil, will flow outward away from the fuser and will therefore not be in place for subsequent starts of the fuser. A less viscous lubricant typically is not retained for an extended period on the bell because of movement and because of the effects of heat.
U.S. Pat. No. 5,852,763 to Okuda et al. is illustrative of belt fusers to which this invention is directed and discusses relevant belts and the grease in some detail. However, it does not recognize that the high viscosity of the grease in a cold condition results in excessive forces on the outer, release surface resulting in significant wear. This invention is direct to reducing damage from such wear.
U.S. Pat. No. 5,300,999 to Koh et al. Also discusses lubrication of a belt fuser with regard to heating operation and grease viscosity. Temperature detection is coupled to a time delay scheme in order to lower grease viscosity by delaying start up torque. The patent also does not recognize that the high viscosity of the grease in a cold condition results in excessive forces on the outer, release surface, resulting in significant wear. Since this patent teaches replenishment of its lubricant during use, the lubricant used is apparently of low viscosity or low stability.
Known prior art greases for belt fusers include perfluoro-(ethylene oxide/methylene oxide) copolymer thickened with 0.2 micron spherical polytetrafluoroethylene particles. This invention employs a different perfluoro polymer which, when used in a system having preheating, achieves full function without significant wear on the belt or loss of the lubricant.